Hitherto, as the element member of semiconductor device, photosensitive device for use in electrophotography, image input line sensor, image pickup device, or other optical devices, there have been proposed a number of amorphous semiconductor films, for example, an amorphous deposited film composed of a silicon containing amorphous material compensated with hydrogen atom or/and halogen atom such as fluorine atom or chlorine atom [hereinafter referred to as "A-Si(H,X)"]. Some of such films have been put to practical use.
Along with these amorphous semiconductor films, there have been proposed various method for their preparation using plasma chemical vapor deposition technique wherein a raw material is decomposed by subjecting it to the action of an energy of direct current, high frequency or microwave glow discharging to thereby form a deposited film on a substrate of glass, quartz, heat-resistant resin, stainless steel or aluminum. There have been also proposed various apparatus for practicing such methods.
Now, in recent years, the public attention has been focused on plasma chemical vapor deposition process by means of microwave glow discharging [hereinafter expressed by the abbreviation "MW-PCVD process"] also at industrial level.
One representative apparatus for practicing such an MW-PCVD process is that which has a structure as shown in the schematic perspective drawing of FIG. 2.
In FIG. 2, there are shown a whole vacuum chamber 1, a microwave introducing window 2 which is made of a dielectric material such as alumina ceramics or quartz, a waveguide 3 which propagates microwave 4 generated from a microwave power source (not shown), an exhaust pipe 5 being connected through an exhaust valve (not shown) to an exhaust apparatus (not shown), a substrate 6 onto which a deposited film is to be formed and a substantially enclosed deposition space 7 (plasma generating space).
The film forming operation in the above-mentioned apparatus is carried out in the following way.
That is, the air in the vacuum chamber 1 is evacuated by opening the main valve of the exhaust pipe 5 to bring the deposition space of the vacuum chamber to a predetermined vacuum. A heater (not shown) installed in a substrate holder (not shown) is actuated to uniformly heat the substrate 6 to a predetermined temperature and to maintain that temperature.
Then, raw material gases, for instance, silane gas such as SiH.sub.4 gas and hydrogen gas (H.sub.2 gas) etc. in the case of forming a silicon containing amorphous deposited film, are introduced into the deposition space 7 of the vacuum chamber 1 through the gas feeding means (not shown) while maintaining the deposition space at a vacuum of less than 1.times.10.sup.-2 Torr.
Successively, microwave 4 having a frequency, for example, of 2.45 GHz from the microwave power source (not shown) is introduced through an isolator, a power monitor, a stub tuner (these are not shown), then through the wave guide 3 and the microwave introducing window 2 into the deposition space 7. The raw material gases thus introduced into the deposition space 7 are excited and dissociated by an energy of the microwave to generate plasmas and to cause chemical reactions among them resulting in formation of a deposited film on the surface of the substrate 6.
By the way, the plasmas thus generated function as a kind of absorber or reflector for microwave which has a tendency to space-propagate within a dielectric medium. The density of the plasmas becomes reduced as the vacuum degree is raised because the mean free path of charged particles becomes longer. Because of this, when the vacuum degree at the time of forming plasmas is high, it becomes possible to make the propagation length of the microwaves longer.
However, in the case where a deposited film is intended to form utilizing mainly neutral radical particles, the lower limit of the vacuum degree lies in the order of 10.sup.-3 Torr. Therefore, in the case where a deposited film is intended to form on a large surface area substrate such as a cylindrical substrate (drum) for use in electrophotography, it is difficult to generate plasmas uniformly all over the surface of such drum. In this respect, there is employed the type of apparatus using the MW-PCVD process as shown in FIG. 2 in which microwave energies with the same resonant mode are introduced from both the upper and lower sides of the drum to thereby generate plasmas.
In such known MW-PCVD apparatus, the relative setting angle between the waveguides 3 connected respectively to the upper and lower microwave introducing windows 2 which resonate with TE.sub.11 mode is made 0.degree.. Because of this, there is a problem that the microwaves introduced often adversely penetrate the respective waveguides opposite each other and cause undesired damage on the respective isolators. There is also another problem that those microwaves which remain not polarized sufficiently by the magnetic field of the isolator penetrate the respective magnetrons of the microwave power sources and interfere with the microwaves therefrom to thereby interfere with the oscillation of microwave.